File concatenation principle

docs/project/principles/README.md

@@ -27,3 +27,4 @@ wants to pursue, as well as those we want to exclude.
 -   [Prefer providing only one way to do a given thing](one_way.md)
 -   [One static open extension mechanism](static_open_extension.md)
 -   [Success criteria](success_criteria.md)
+-   [File concatenation](file_concatenation.md)

docs/project/principles/file_concatenation.md

@@ -0,0 +1,49 @@
+# Principle: File concatenation should preserve meaning
+
+<!--
+Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+Exceptions. See /LICENSE for license information.
+SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+-->
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Abstract](#abstract)
+-   [Background](#background)
+-   [Principle](#principle)
+-   [Applications of these principles](#applications-of-these-principles)
+
+<!-- tocstop -->
+
+## Abstract
+
+It's always possible to concat together Carbon source files without changing the meaning of or diagnostics applied to the code.
+
+## Background
+
+In C++, it is a common practice to concatenate files together in order to
+recreate and distribute a bug repro or to build a piece of C++ code in an
+environment without the requirement of a build system. This allows code taken
+from any build environment to be easily compiled by a developer in any other
+environment, through a single compiler invocation.
+
+The reason this works is largely historical, as #include performs a textual
+inclusion, but it can be maintained without textual includes as well.
+
+## Principle
+
+Concatenating any two Carbon files together will not change the meaning of the
+code in either file. It will be required to order the concatenation so that
+dependencies are ordered first, in order to maintain the information
+accumulation principle. There is always an ordering that allows any Carbon
+project to be fully concatenated into a single Carbon source file and thus built
+with a single compiler invocation.
+
+## Applications of these principles
+
+Carbon uses syntax to denote the start of a library or package, which functions
+at the top of a file or in the middle of a file in the same way. Language rules
+may change behaviour and/or diagnostics based on the relationship of code
+between libraries or packages, but not between files.